The metal working process known as extrusion involves pressing metal stock (ingot or billet) through a die opening having a predetermined configuration in order to form a shape having a longer length and a substantially constant cross-section. For example, in the extrusion of aluminum alloys, the aluminum stock is preheated to the proper extrusion temperature. The aluminum stock is then placed into a heated cylinder. The cylinder utilized in the extrusion process has a die opening at one end of the desired shape and a reciprocal piston or ram having approximately the same cross-sectional dimensions as the bore of the cylinder. This piston or ram moves against the aluminum stock to compress the aluminum stock. The opening in the die is the path of least resistance for the aluminum stock under pressure. The aluminum stock deforms and flows through the die opening to produce an extruded product having the same cross-sectional shape as the die opening.
Referring to FIG. 1, the foregoing described extrusion process is identified by reference numeral 2, and typically consists of several discrete and discontinuous operations including: melting 4, casting 6, scalping 8, homogenizing 10, optionally sawing 12, reheating 14, and finally, extrusion 16. The aluminum stock is cast at an elevated temperature and typically cooled to room or ambient temperature. After casting, the aluminum stock is scalped to remove the oxide layer that naturally forms on the surface of the aluminum stock due to the reaction between the aluminum surface and the oxygen in the atmosphere. Because the aluminum stock is cast, there is a certain amount of inhomogeniety in the structure of the aluminum stock. Therefore, the aluminum stock is typically heated at elevated temperatures to homogenize the cast metal. Following the homogenization step, the aluminum stock is cooled to room temperature. After cooling, the homogenized aluminum stock is reheated in a furnace to an elevated temperature called the preheat temperature. Those skilled in the art will appreciate that the preheat temperature is generally the same for each billet that is to be extruded in a series of billets. Upon reaching the preheat temperature, the aluminum stock is placed in an extrusion press and extruded through the extrusion die to form an extruded product.
All of the foregoing steps relate to practices that are well known to those skilled in the art of casting and extruding. Each of the foregoing steps is related to metallurgical control of the metal to be extruded. These steps are very cost intensive, with energy costs incurring each time the metal stock is reheated from room temperature. There are also in-process recovery costs associated with the need to trim the metal stock, labor costs associated with process inventory, and capital and operational costs for the extrusion equipment.
Therefore, there exists a need to consolidate the discrete and discontinuous operations of a traditional extrusion process to reduce the cost of manufacturing an extruded product.
Previous attempts to develop a continuous extrusion process are described in U.S. Pat. Nos. 6,536,508, 6,712,126 and 6,739,485 by Sample et al. These patents are incorporated by reference. Also, these patents describe a system for extruding an article in a continuous fashion accomplished by using multiple injectors of molten metal operating sequentially. Each of the injector is connected between the source of molten metal and the downstream process. Accurate synchronization is required between the multiple injectors for successful operation. The synchronization is achieved by means of valves that open or close to facilitate or impede the flow of molten aluminum. The reliability and ease of operation of these valves is crucial to the success of these inventions.
While these patents provide a continuous process it is desirable to provide an apparatus and continuous method of extrusion that consolidates the multiple operations of a traditional extrusion process into a single operation. The operation of the invention disclosed here is significantly more reliable than previous inventions to achieve the same goal. The improved reliability is a result of the simplification of certain components and due to the invention of additional components that reduce the complexity of tasks involved in continuously extruding an article.